Method for producing an optical scale

ABSTRACT

A length of scale ( 1 ) which has a profile consisting of a sequence of troughs ( 18 ) and crests ( 16 ) is subjected to a process to make the shapes of the crests more regular, thus increasing and making more predictable the signal reflected from the crests on the scale. The process comprises passing a length of scale ( 1 ) between upper ( 20 ) and lower ( 22 ) cylindrical rollers. The lower roller ( 22 ) may be driven or the length of scale ( 1 ) may be pulled through. Reference marks or identification marks may be added to the scale by appropriately marking the upper roller ( 20 ). Alternatively, the marks may be added at an initial embossing step which imparts the troughs and crests profile to the scale or at a separate subsequent step.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to producing a scale for use inopto-electronic scale reading apparatus.

2. Description of Related Art

A known form of opto-electronic scale reading apparatus for measuringrelative displacement of two members comprises a scale on one of themembers, having scale marks defining a periodic pattern, a read headprovided on the other member, means for illuminating the scale, periodicdiffraction means for interacting with light from the scale marks toproduce interference fringes having movement relative to the read headand detecting means in the read head responsive to the fringes toproduce a measure of the displacement.

An example of such apparatus is disclosed in EP-A-0 207 121 and alsoU.S. Pat. No. 4,974,962, each of which shows the means for illuminatingand the periodic diffraction means in the read head. U.S. Pat. No.4,926,566 discloses a method of producing a scale, in the form of aflexible tape produced by rolling, the pitch of the scale marks being 20μm or 40 μm for example. The illuminating means, the diffraction meansand the detecting means responsive to the fringes may be integrated inthe read head in the manner described in U.S. Pat. No. 5,302,820. Such ascale may have reference marks along its length, as disclosed inWO94/25830 in which a depression is provided on the scale by stampingthe scale at appropriate intervals with a cylindrical roller.

In the production of a length of scale, for example as described in U.S.Pat. No. 4,926,566, although flanks of the embossed profile of the scaleare well defined, the crests can be somewhat irregular. Although this isnot a problem in scale reading apparatus in which the scale is lit froman end, since the crests are not read, this could be a problem inapparatus in which the scale is lit from one side, in which the crestsare read.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of producing a length of scale for use in opto-electronic scalereading apparatus, comprising subjecting a length of scale to anembossing process to form a profile which is a sequence of troughs andcrests, wherein after the embossing process the length of scale issubjected to a further process which acts on the crests to make moreregular their shapes.

The further process could comprise passing the embossed length of scalebetween first and second rollers to render the crests more regular asregards their shapes. In this case, one of the rollers could be a drivenroller to pass the length of scale between the rollers. Alternatively,the length of scale could be pulled through the rollers. Each of therollers could be a plain roller.

In a second aspect of the invention, there is provided a method ofproviding reference marks or identification marks on the scale duringthe embossing process or the process which acts on the scale to maketheir shapes more regular.

The reference marks or identification marks may be imprinted on thescale in either process by providing the relevant roller with suitablemarks.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of an embossing process on alength of scale;

FIG. 2 is a diagrammatic illustration of two rollers acting on a lengthof scale;

FIG. 3 shows the rollers of FIG. 2 with laser etching on the upperroller;

FIG. 4 shows the rollers of FIG. 2 with embossing means on the upperroller; and

FIG. 5 shows the profile of a finished scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a length of scale 1 for use in opto-electronic scale readingapparatus is produced, for example, by a method as described in U.S.Pat. No. 4,926,566, in which the scale is passed between upper and lowerrollers 10,12 as shown in FIG. 1, in which the upper roller 10 has aprofile 14 which is the counterpart of the desired scale profile. Thelower roller 12 may be plain, or it may also have a profile, asdescribed in U.S. Pat. No. 5,088,209. This embossing process results ina scale profile which is a sequence of troughs 18 and crests 16.Typically, the scale could be of spring steel over which is provided alayer of copper. This may be plated with nickel and/or gold, asdescribed in U.S. Pat. No. 4,926,566. The sequence of troughs and crestsprovide marks which can be read optically in such apparatus.

Next as shown in FIG. 2, the embossed length of scale 1 is passedbetween an upper cylindrical roller 20 and a lower, backing cylindricalroller 22, the latter, for example, being a driven roller. By way ofexample, roller 20 could be made up of a steel core with a smoothelectroless nickel coating. Instead of being passed between the rollers20 and 22 by the driving of roller 22, the length of scale 1 could bepulled through the rollers.

The effect of the above process is to make more regular the shapes ofthe crests of the profile of the length of scale 1 as regards theirshapes, increasing and making more predictable the signal achieved byreflection from the crests in use of the length of scale andsubstantially eliminating plating imperfections and disturbances fromthe earlier embossing process. For example, the crests 16 may beflattened as shown at 30 in FIG. 5. Such flattened crests give improvedlight reflection, even when lit from the side.

Typically, the upper cylindrical roller 20 is a plain roller. The lowercylindrical roller 22 is either a plain roller or a profiled roller, aswith the roller 12. The operation could instead be done with a suitablymarked top roller which leaves appropriate marks on the scale crests.These might simply be a part number or other identification, but equallycould be used to perform some function, like providing referencemarking, providing for absolute position encoding or providing forautomatic read head configuration.

It should be noted that this further process should have no substantialpermanent effect on the pitch of the scale so as not to disturb thescale accuracy.

The marks may be applied to the crests of the scale by laser etching thedesired marks 24 in reverse on the upper roller as in FIG. 3. On passingthe embossed scale through the rollers, the mark is imprinted onto thescale crests, each revolution of the roller producing a repeat of themark on the scale. This method is particularly suitable for addingidentification numbers etc on the scale.

Reference marks may also be added to the scale by incorporating anembossing device with the upper roller, such as a flat-ended chisel 26as shown in FIG. 4. As the chisel hits the scale it flattens some of thecrests and creates a reflective feature 28 on the scale, such as abright plane mirror.

Another method of producing marks on the scale is by chemical etching.For example using a spring steel scale coated with first copper and thengold, some of the gold outer layer is removed mechanically to expose thecopper underneath. The exposed copper is chemically treated to blackenit, for example using sodium sulphate. A 200 μm-wide dark line producedby this method in a 40 μm period incremental scale results in that theincremental signal only suffers a 20% drop in size as it passes thereference mark. This is a result of the filtering effect of thereadhead, as described in EP 207121.

The marks could be added to the scale in a similar manner at other stepsof the scale-making process, such as a subsequent step or the initialembossing step. Adding the reference marks in the subsequent step hasthe advantage that a stamping device other than a roller may be usedallowing the reference marks to be added in an irregular pattern. Ascale which has been marked in the initial embossing step may be passedthrough the plain rollers with very little effect on the scale.

Where the mark is an identification number, it may be imprinted on thefill width of the scale or alternatively just on a fraction of the widthof the scale (i.e. along one edge).

Reference marks on the scale may be single, periodic or in specificsequences. A pseudorandom pattern of dark or bright lines of differentwidths can be formed in the longitudinal direction of the scale to makean autocorrelator reference mark that can be detected with a matchingdetector pattern. A series of bright and dark lines in the scale can bearranged to form a zone plate that will act as a lens and can be used toimage a reference pattern onto a matching detector. A pattern of markscan also be provided (by the above methods) which encodes informationabout the absolute position along the scale.

Any of the above methods of marking the scale may be used independently,even without the further process for making the crests 16 more regular.

What is claimed is:
 1. A method for producing a scale for anopto-electronic scale reading apparatus comprising: subjecting a lengthof scale (1) to an embossing process to form a profile which is asequence of troughs (18) and crests (16); characterised in that afterthe embossing process, the length of scale is subjected to a furtherprocess which acts on the crests to make their shapes more regular.
 2. Amethod for producing a scale according to claim 1, wherein said furtherprocess comprises passing the length of scale between an uppercylindrical roller and a lower backing cylindrical roller.
 3. A methodfor producing a scale according to claim 2, wherein the lower backingcylindrical roller (22) is a driven roller.
 4. A method for producing ascale according to claim 2, wherein the length of scale (1) is pulledthrough the rollers (20,22).
 5. A method for producing a scale accordingto claim 2, wherein the upper roller (20) is made of a steel core with asmooth electroless coating.
 6. A method for producing a scale accordingto claim 2, wherein the upper roller (20) is suitably marked to leaveappropriate marks on the scale.
 7. A method of producing a scaleaccording to claim 6, wherein the upper roller (20) is marked by laseretching.
 8. A method of producing a scale according to claim 6, whereinthe upper roller (20) is suitably marked to selectively remove an outerlayer of the scale and the scale is chemically treated to blacken themarks in the scale.
 9. A method for producing a scale according to claim6, wherein the upper roller (2) is marked with embossing means.
 10. Amethod for producing a scale according to claim 9, wherein the embossingmeans comprise at least one flat-ended chisel (26).
 11. A method forproducing a scale according to claim 1, wherein marks are added to thescale during the embossing process, the embossing process comprisingpassing the scale through upper (10) and lower (12) rollers and theupper roller (10) being suitably marked to produce said marks.
 12. Amethod for producing a scale according to claim 11, wherein the upperroller (10) is marked by laser etching.
 13. A method for producing ascale according to claim 11, wherein the upper roller (10) is markedwith further embossing means.
 14. A method for producing a scaleaccording to claim 13, wherein the further embossing means comprises atleast one flat-ended chisel.
 15. A method for producing a scaleaccording to claim 11, wherein the upper roller is suitably marked toselectively remove an outer layer of the scale and the scale is thenchemically treated to blacken the mark produced on the scale.
 16. Amethod for producing a scale according to claim 1, wherein marks areadded to the scale in a subsequent step, following the further processwhich acts on the scale to make their crests more regular.
 17. A methodfor producing a scale according to claim 16, wherein the subsequent stepcomprises mechanically selectively removing an outer layer of the scaleand then chemically treating the scale to blacken the marks produced onthe scale.
 18. A method for producing a scale according to claim 16,wherein the subsequent step comprises using further embossing means toemboss a mark in the scale.
 19. A method for producing a scale accordingto claim 18, wherein the further embossing means comprises a flat-endedchisel.